unity8: initial packaging (!27)

* Mir starts up and is able to display system settings
* x86_64 only for now, because at least ubuntu-app-test did not build
  on aarch64

Based on PureTryOut's work. Getting it to this stage was a huge effort
(as it shows in the package count: 111(!)). See the merge request for
details.

[skip ci]: this won't finish in CI; ollieparanoid made sure that
           everything builds for x86_64.

main/dbus-cpp/boost_io_context.patch

@@ -0,0 +1,25 @@
+diff --git a/include/core/dbus/asio/executor.h b/include/core/dbus/asio/executor.h
+index d1f043e..505a55b 100644
+--- a/include/core/dbus/asio/executor.h
++++ b/include/core/dbus/asio/executor.h
+@@ -18,18 +18,12 @@
+ #ifndef CORE_DBUS_ASIO_EXECUTOR_H_
+ #define CORE_DBUS_ASIO_EXECUTOR_H_
+ 
++#include <boost/asio/io_service.hpp>
++
+ #include <core/dbus/bus.h>
+ #include <core/dbus/executor.h>
+ #include <core/dbus/visibility.h>
+ 
+-namespace boost
+-{
+-namespace asio
+-{
+-class io_service;
+-}
+-}
+-
+ namespace core
+ {
+ namespace dbus